1. Field of the Invention
The present invention relates to a semiconductor switching apparatus and, more particularly, to a power semiconductor switching apparatus.
2. Description of the Related Art
A semiconductor switching apparatus has been widely used as a switch for a power circuit. As an example of such a semiconductor switching apparatus, a power semiconductor switching apparatus for use in a power circuit in which a current of several hundreds A (ampere) or more flows is schematically shown in FIG. 1. This semiconductor switching apparatus comprises an anode electrode 1 made of copper and serving as a heat sink and a cathode electrode 2 made of copper and serving as a heat sink. Two metal layers 4 are formed in contact with the anode and cathode electrodes 1 and 2, respectively. A semiconductor chip 3 constituting a semiconductor switching element such as a thyristor or a GTO is formed between the layers 4. The chip 3 is in contact with the layers 4. A gate electrode 5 is connected to the chip 3. The switching apparatus is covered with a case (not shown). When this semiconductor switching apparatus is completed as a product, forces are externally applied on the anode and cathode electrodes 1 and 2 in directions indicated by arrows. Therefore, the anode and cathode electrodes 1 and 2, the semiconductor chip 3 and the metal layers 4 are urged against each other. This is to sufficiently decrease a thermal resistance induced between the chip 3 and the electrodes 1 and 2 due to heat generated in the chip when the switching apparatus is used in a power circuit. A large thermal stress is generated in the chip 3 due to a difference between thermal expansion coefficients of the electrodes 1 and 2 and the chip 3. The layers 4 are formed in order to reduce this thermal stress.
A method of urging and mounting a semiconductor element is disclosed in Japanese Patent Disclosure (Kokai) No. 53-143174. In this prior art, a disk-like thyristor is used. Thermally conductive fins are arranged at both sides of the thyristor. A pair of a bolt and a nut are located around the thyristor to extend through the fins at the both sides of the thyristor. An interval between the fins is narrowed by tightening the bolt and nut, thereby applying a force on the thyristor. As a result, the thyristor as a semiconductor element is urged and mounted by the fins. Another mounting method is disclosed in Japanese Patent Disclosure (Kokai) No. 54-89574. This prior art discloses a tightening apparatus for an electronic semiconductor element. This apparatus employs a flat thyristor as a semiconductor element. Cooling members are arranged at both sides of the thyristor. A projection extends outward from a central portion of one cooling member. An insulator is located at a side portion of the thyristor. A leaf spring is located outside the cooling member having the projection. Holes for receiving a bolt are formed in the two cooling members, the insulator and the leaf spring. The bolt is inserted in the holes. A nut is threadably engaged with the bolt from outside the leaf spring. When the nut is brought into contact with the leaf spring, a force is applied on the leaf spring. When the leaf spring is brought into contact with the projection of the cooling member, the force is applied on the cooling member. In this manner, the force is applied on the thyristor located between the cooling members. As a result, the thyristor is urged against the cooling members located at its both sides.
In the conventional power semiconductor switching apparatus shown in FIG. 1, in order to increase a current capacity, the diameter of the semiconductor chip 3 must be increased. For this purpose, a large semiconductor wafer is required. When the current capacity of the switching apparatus is increased, heat generated from the chip is increased. Therefore, a means for efficiently radiating the heat and a means for reducing a stress induced due to the heat are necessary. In order to improve switching characteristics of the semiconductor switching apparatus, a semiconductor wafer must be micropatterned. It is very difficult to perform micropatterning within the entire large semiconductor wafer with equal precision. For this reason, if a structure of the conventional semiconductor switching apparatus is adopted, it is difficult to manufacture a power semiconductor switching apparatus having a large capacity and good switching characteristics.
In addition, in the conventional semiconductor switching apparatus, heat is radiated from an electrode surface of the semiconductor chip 3. For this reason, in order to sufficiently radiate the heat from the chip 3, a relatively large urging force must be externally applied on the switching apparatus. The urging force, however, is not always in proportion to a heat radiation efficiency. Therefore, it is meaningless to apply a very large urging force on the switching apparatus. Also, if a very large urging force is applied between the anode and cathode, it becomes difficult to reduce the thermal stress induced in the semiconductor chip 3, and the chip itself may be damaged. In the conventional switching apparatus, each member may be broken due to the thermal stress because the urging force is applied. As a result, according to the conventional semiconductor switching apparatus, a cooling performance is limited, and it is difficult to obtain a large capacity.